1. Field of the Invention
This invention relates to optical data transmission over optical fibers. Specifically, and not by way of limitation, the present invention relates to optical data transmission utilizing coherent optical channel substitution.
2. Description of the Related Art
Information has been transmitted over optical fibers for some time. Details about this field are disclosed in “Optical Communication Systems,” by J. Gowar (Prentice Hall, 2nd ed., 1993) and “Fiber-optic communication systems” by G. P. Agrawal (Wiley, 2nd ed., 1997), which are herein incorporated by reference. The information is usually in the form of binary digital signals, i.e. logical “1”s and “0”s, but fiber optics is also used to transport analog signals, such as cable TV signals. The present invention may be utilized with either digital or analog signals. However, for simplicity and exemplary purposes only, digital signals are discussed with the present invention. It should be understood by those skilled in the art that the present invention may also be used with analog signals. Every optical data transmission system has a transmitter, which emits light modulated with information into the fiber, and a receiver at the far end which detects the light and recovers the information. A long distance digital link may also use one or more digital regenerators at intermediate locations. A digital regenerator receives a noisy version of the optical signal, makes decisions as to what sequence of logical values (“1”s and “0”s) was transmitted, and then transmits a clean noise-free signal containing that information forward towards the destination.
As discussed, in Agrawal, an optical signal that is modulated has a broadened spectrum occupying a range of frequencies. Consider propagation of Gaussian input pulses in optical fibers having the amplitude
      A    ⁡          (      t      )        =            A      0        ⁢          exp      ⁡              (                              -                                          1                +                                  i                  ⁢                                                                          ⁢                  C                                            2                                ⁢                                    (                              t                                  T                  0                                            )                        2                          )            where A0 is the peak amplitude. The parameter T0 represents the half-width at 1/e-intensity point. The parameter C governs the linear frequency chirp imposed on the pulse. A pulse is said to be chirped if its carrier frequency changes with time. The Fourier spectrum of a chirped pulse is broader than that of the unchirped pulse. This can be seen by taking the Fourier transform of the equation above for A(t), so that
            A      ~        ⁡          (      ω      )        =                              A          0                ⁡                  (                                    2              ⁢              π              ⁢                                                          ⁢                              T                0                2                                                    1              +                              i                ⁢                                                                  ⁢                C                                              )                            1        /        2              ⁢          exp      ⁡              (                  -                                                    ω                2                            ⁢                              T                0                2                                                    2              ⁢                              (                                  1                  +                                      i                    ⁢                                                                                  ⁢                    C                                                  )                                                    )            The spectral half-width (at 1/e-intensity point) is given by
      Δω    0    =                    (                  1          +                      C            2                          )                    1        /        2              ⁢          T      0              -        1            In the absence of frequency chirp (C=0), the spectral width satisfies the relation Δω0T0=1.
As described herein, if an optical signal is said to carry information then that information is useful to a recipient of the optical signal. However, there are other disclosures which employ a broader definition, where any optical signal that varies with time is said to carry information. The broader definition does not apply in the explanation of the present invention. The variations of the optical signal with time must convey a message that is of value to a recipient in order for the optical signal to contain information.
In the 1990's optical amplifiers were deployed in telephony and cable TV networks, in particular erbium doped fiber amplifiers (EDFAs) were deployed. These devices amplify the optical signals passing through them, and overcome the loss of the fiber without the need to detect and retransmit the signals. A typical long distance fiber optic digital link might contain some digital regenerators between the information source and destination, with several EDFAs in between each pair of digital regenerators.
A. WDM Network Topologies and Add-drop
Also in the 1990's, wavelength division multiplexing (WDM) was commercially deployed, which increased the information carrying capacity of the fiber by transmitting several different wavelengths in parallel.
There are several different topologies that may be employed in a WDM fiber optic network, which are illustrated in FIGS. 1a through e. The dashed, dotted and solid lines in FIGS. 1a through e represent three different wavelengths. The simplest topology is point-to-point transmission, shown in FIG. 1a, where all the wavelengths originate in the same location A (101) and terminate together at another location B. As shown in FIG. 1b, broadcast topologies are possible where a signal from one transmitter is split and goes to more than one receiver (locations B (102) and C (103) in FIG. 1b). In a WDM transmission system, one WDM channel can be dropped at an intermediate site while the others continue, as shown in FIG. 1c. The dropped channel 104 may be detected at that intermediate site (node D (105) in FIG. 1c). Alternatively, as shown in FIG. 1d, the dropped channel may traverse more fiber spans (from D (105) to E (106)) before being detected. The process of add-drop, illustrated in FIG. 1e, means that as well as dropping a WDM channel 104 of a certain wavelength at an intermediate site 107, a channel 108 of the same wavelength carrying new content is inserted and continues with the other channels. The added and dropped channels may originate and terminate at the add-drop node, as is shown in FIG. 1c or they may originate and terminate at a remote location, from the add-drop node, in analogy with FIG. 1d. To make an add-drop node (node D (105) in FIG. 1e), it is necessary to block light coming from the transmitter at A at the add-drop wavelength so it does not continue to B, and pass all other wavelengths going from A to B. U.S. Pat. No. 5,748,349 gives an example of the apparatus to perform the add-drop function. A high extinction is required for the blocking operation because the crosstalk onto the channel added at location D is in-band crosstalk, which causes more degradation to a signal than out-of-band crosstalk. Except for the broadcast case, fiber optic links are typically bidirectional and symmetric, so another channel, often in another fiber, is transmitted from the receiver site to the transmitter site.
Most fiber optic transmission systems installed today have static add-drop configurations. There has been substantial research into all-optical networks where connections between nodes are set up and taken down in an automated fashion according to demand. All-optical networks are described in “A Precompetitive Consortium on Wide-Band All-Optical Networks,” by S. B. Alexander et al. (IEEE J. Lightwave Technol., vol. 11, no. 5/6, p. 714-735, 1993) which is incorporated herein by reference. The add-drop technology must be able to switch multiple wavelength channels from add-drop to passthrough. Also the components involved in signal transmission, that is transmitters and receivers, fiber spans, optical amplifiers, etc., must be able to support a wide range of possible end-to-end link scenarios, as the connections in an all-optical network are changed according to customer demand. Using the add-drop technology to switch in a digital regenerator at intervals along the link helps support long transmission distance scenarios. Hence, there is a need for a flexible add-drop technology that may be switched in and out at any one wavelength and which may be operated at a range of wavelengths. Also there is a need for a digital regenerator which may be switched in and out of a link and which may be operated at a range of wavelengths.
B. Direct Detection & Coherent Detection
The transmitter unit for a single WDM channel contains a light source, usually a single longitudinal mode semiconductor laser. Information is imposed on the light by direct modulation of the laser current, or by external modulation, that is by applying a voltage to a modulator component that follows the laser. The receiver employs a photodetector, which converts light into an electric current. There are two ways of detecting the light: direct detection and coherent detection. All the installed transmission systems today use direct detection. Although it is more complex, coherent detection has some advantages, and it was heavily researched into in the 1980s and the start of the 1990s, and has become of interest once again in the past few years.
Most deployed transmission systems impose information on the amplitude (or intensity, or power) of the signal. The light is switched on to transmit a “1” and off to transmit a “0”. In the case of direct detection, the photodetector is presented with the on-off modulated light, and consequently the current flowing through it is a replica of the optical power. After amplification the electrical signal is passed to a decision circuit, which compares it to a reference value. The decision circuit outputs an unambiguous “1” or “0”.
There is another class of modulation formats where information is encoded on the phase of the optical signal, such as optical differential phase shift keying (oDPSK). A photodetector does not respond to changes in the phase of the light falling on it, so a passive component called a discriminator is used before the photodetector receives the optical signal. The discriminator converts the changes in phase into changes in power to which the photodetector can respond.
Since a photodetector does not respond to the phase portion of an optical wave, if two wavelengths are input to the photodetector, the photodetector does not distinguish between them. The photocurrent is proportional to the sum of the powers of the two wavelength channels. WDM systems work by using passive optical filter components to separate out the different wavelength channels at the receive terminal, so each photodetector sees only one channel. This approach puts a limit on how close the channels can be spaced, which comes from the optical filter's ability to pass one channel and reject its neighbours.
The coherent detection method treats the optical wave more like radio, inherently selecting one wavelength and responding to its amplitude and phase. “Fiber-optic communication systems” by G. P. Agrawal provides an introduction to coherent detection. Coherent detection involves mixing the incoming optical signal with light from a local oscillator (LO) laser source. FIG. 2 illustrates an example of a coherent receiver suitable for detecting a binary phase shift keyed (BPSK) signal. The incoming signal 201 is combined with light 202 from a continuous wave (c.w.) local oscillator in a passive 2:1 combiner 203. The LO light has close to the same state of polarisation (SOP) as the incoming signal and either exactly the same wavelength (homodyne detection) or a nearby wavelength (heterodyne detection). When the combined signals are detected at photodetector 204, the photocurrent contains a component at a frequency which is the difference between the signal and local oscillator optical frequencies. This difference frequency component, known as the intermediate frequency (IF), contains all the information, that is amplitude and phase, that was on the optical signal. Because the new carrier frequency is much lower, typically a few gigahertz instead of 200 THz, all information on the signal can be recovered using standard radio demodulation methods. Coherent receivers see only signals close in wavelength to the local oscillator, and so by tuning the LO wavelength, a coherent receiver can behave as though having a built-in tunable filter. When homodyne detection is used, the photocurrent is a replica of the information and can be passed to the decision circuit 206 which outputs unambiguous “1” or “0” values. With heterodyne detection, the photocurrent must be processed by a demodulator 205 to recover the information from the IF. FIG. 2 illustrates a configuration for single-ended detection. There are other configurations for coherent detection. For example, a balanced detection configuration is obtained by replacing the 2:1 combiner by a 2:2 combiner, each of whose outputs are detected and the difference taken by a subtracting component.
Following is a mathematical description of the coherent detection process. (The complex notation for sinusoids is summarised in the Appendix.) The electric field of the signal may be written asRe└Es(t)eiωst+iφs(t)┘where Es(t) is the slowly varying envelope containing the information encoded on amplitude and phase of the optical signal, ωs is the angular frequency of the optical carrier, and φs(t) is the slowly varying phase noise associated with the finite linewidth of the laser. Writing the phase noise separate from the modulation envelope Es(t) has the advantage that in the case of digital information transmission Es(t) takes on only a small number of possible values, depending on the digital signal format. Similarly, the electric field of the local oscillator is written asRe└ELOeiωLOt+iφLO(t)┘where ELO is a constant given that the local oscillator is c.w., ωLO is the angular frequency of the LO, and φLO(t) is the phase noise on the LO. The electric fields of the signal and LO are written as scalar quantities because it is assumed that they have the same state of polarisation. The electric field of the light arriving at the photodetector in FIG. 2 is the sum of the two electric fieldsE1=Re└Es(t)ei(ωst+φs(t))+ELOei(ωLOt+φLO(t))┘and the optical power isP1=E1*E1 P1=|Es(t)|2+|ELO|2+2Re[Es(t)ELO*ei(ωs−ωLO)t+i(φs(t)−φLO(t))]  (1)In the case of single ended detection only one output of the combiner is used. |ELO|2 is constant with time. |Es(t)|2 is small given that the local oscillator power is much larger than the signal power, and for phase shift keying (PSK) and frequency shift keying (FSK) modulation formats |Es(t)|2 is constant with time. The dominant term in equation 1 is the beat term Re└Es(t)ELO*ei(ωs−ωLO)t+i(φs(t)−φLO(t))┘. In appropriate conditions the beat term can be readily obtained from the photocurrent in the single-ended detection case. Alternatively when |Es(t)|2 is not small and varies with time, the beat term is produced directly by the balanced detection configuration. The equations that follow refer to the beat term. It is assumed that this term is obtained by single ended detection assuming the other terms do not contribute or by balanced detection.
There are two modes of coherent detection: homodyne and heterodyne. In the case of homodyne detection the frequency difference between signal and local oscillator is zero, and the local oscillator laser has to be phase locked to the incoming signal in order to achieve this. For homodyne detection the term ei(ωs−ωLO)t+i(φs(t)−φLO(t)) is 1, and the beat term becomesRe└Es(t)ELO*┘For the binary phase shift keying (BPSK) modulation format for example, Es(t) takes on the value 1 or −1 depending on whether a logical “1” or “0” was transmitted, and the decision circuit can simply act on the beat term directly.
With heterodyne detection there is a finite difference in optical frequency between the signal and local oscillator. All the amplitude and phase information on the signal appears on a carrier at angular frequency (ωs−ωLO), the intermediate frequency, and it can be detected with a demodulator using standard radio detection methods, such as synchronous detection, envelope detection or differential detection. Typically homodyne detection gives better performance than heterodyne detection, but is harder to implement because of the need for phase locking.
C. Sampled Coherent Detection
A new method of coherent detection called sampled coherent detection has been proposed and demonstrated recently, as described in U.S. Patent Application No. 2004/0114939 and in “Coherent detection method using DSP for demodulation of signal and subsequent equalization of propagation impairments” by M. G. Taylor (IEEE Phot. Tech. Lett., vol. 16, no. 2, p. 674-676, 2004) which are herein incorporated by reference. Digital signal processing (DSP) is employed in this method to obtain the information carried by a signal from the beat products seen at the outputs of a phase diverse hybrid. The field of digital signal processing is summarized below.
In sampled coherent detection, the signal and local oscillator are combined in a passive component called a phase and polarisation diverse hybrid. FIG. 3 shows a sampled coherent detection apparatus. The four outputs of the phase and polarisation diverse hybrid are detected by separate photodetectors 312 and then, after optional amplification by amplifiers 313, they are sampled by A/D converters 314. The sample values of the A/D converters are processed by the digital signal processor 315 to calculate the complex envelope of the signal electric field over time. The phase and polarisation diverse hybrid has four outputs 308 through 311 in the example of FIG. 3, where single ended detection is used. The top two outputs 308 and 309 have the LO in one state of polarisation, e.g., the horizontal polarisation, and the lower two outputs 310 and 311 have the LO in the orthogonal, vertical, polarisation. For each of the two LO polarisation states, the signal is combined with the LO in a 90° hybrid 305, also known as a phase diverse hybrid. The phase of the LO relative to the signal in one output of the 90° hybrid is different by π/2 radians (i.e. 90°) compared to the phase of the LO relative to the signal in the other output. This phase shift can be implemented by extra path length in one arm 306 of the 90° hybrid carrying the LO compared to the other arm 305, as can be seen in FIG. 3. The orthogonal SOP relationship between the two 90° hybrids is achieved by using a polarization beamsplitter 304 to divide light from the local oscillator 302 between the two hybrids and a standard 1:2 splitter 303 to divide the incoming signal light 301.
The following mathematical treatment explains how the electric field of the signal is obtained from the outputs of the phase and polarisation diverse hybrid. The incoming signal electric field can be written asRe└Es(t)eiωst+iφs(t)┘where Es(t) is a Jones vector, a two-element vector comprising the polarisation components of the electric field in the horizontal and vertical directions. The use of Jones vectors is summarised in the Appendix.
            E      s        ⁡          (      t      )        =      (                                                      E              sx                        ⁡                          (              t              )                                                                                      E              sy                        ⁡                          (              t              )                                            )  Each of the four outputs of the phase and polarization diverse hybrid in FIG. 3 contains signal Re└Es(t)eiωst+iφs(t)┘. The local oscillator in the four outputs is different, and can be written as followstop output . . . Re└ELOeiωLOt+iφLO(t) {circumflex over (x)}┘2nd output . . . Re└i ELOeiωLOt+iφLO(t) {circumflex over (x)}┘3rd output . . . Re└ELOeiωLOt+iφLO(t) ŷ┘4th output . . . Re└i ELOeiωLOt+iφLO(t) ŷ┘In the top two arms the LO is horizontally polarized, in the direction of Jones unit vector {circumflex over (x)}, and in the lower two arms vertical in the direction of ŷ. The π/2 phase shift is accounted for by the multiplicative imaginary number i. The beat term parts of the optical powers in the four outputs 308 through 311 are thereforebeat term 1=Re└Esx(t)ELO*ei(ωs−ωLO)t+i(φs(t)−φLO(t))┘beat term 2=Im└Esx(t)ELO*ei(ωs−ωLO)t+i(φs(t)−φLO(t))┘beat term 3=Re└Esy(t)ELO*ei(ωs−ωLO)t+i(φs(t)−φLO(t))┘beat term 4=Im└Esy(t)ELO*ei(ωs−ωLO)t+i(φs(t)−φLO(t))┘So the envelope of the signal electric field can be calculated from
                                          E            s                    ⁡                      (            t            )                          =                                            ⅇ                                                                    -                                          ⅈ                      ⁡                                              (                                                                              ω                            s                                                    -                                                      ω                            LO                                                                          )                                                                              ⁢                  t                                -                                  i                  ⁡                                      (                                                                                            ϕ                          s                                                ⁡                                                  (                          t                          )                                                                    -                                                                        ϕ                          LO                                                ⁡                                                  (                          t                          )                                                                                      )                                                                                      E              LO              *                                ⁢                      (                                                                                                      (                                              beat                        ⁢                                                                                                  ⁢                        term                        ⁢                                                                                                  ⁢                        1                                            )                                        +                                          i                      ⁡                                              (                                                  beat                          ⁢                                                                                                          ⁢                          term                          ⁢                                                                                                          ⁢                          2                                                )                                                                                                                                                                                    (                                              beat                        ⁢                                                                                                  ⁢                        term                        ⁢                                                                                                  ⁢                        3                                            )                                        +                                          i                      ⁡                                              (                                                  beat                          ⁢                                                                                                          ⁢                          term                          ⁢                                                                                                          ⁢                          4                                                )                                                                                                                  )                                              (        2        )            To implement equation 2 in the digital signal processor, the frequency difference ωs−ωLO and phase difference φs(t)−φLO (t) must be known. These parameters can be obtained using a standard phase estimation technique, such as described in “Digital Communications” by J. G. Proakis (McGraw-Hill, 4th ed., 2000) and “Digital communication receivers: synchronization, channel estimation & signal processing” by H. Meyr, M. Moeneclaey & S. A. Fechtel (Wiley, 1998).
Transmission over a length of optical fiber transforms the state of polarization of an optical signal, so that the digital values taken on by Es(t) as seen at the receive end of a fiber optic transmission system are typically not the same as those imposed at the transmit end. The polarization transformation can be reversed within the DSP by applying the appropriate rotation Jones matrix. The correct rotation matrix can be found by exploring the available space and then locking on to the matrix which gives the best quality signal. The polarization transformation of the optical fiber typically changes slowly, so the rotation matrix must be allowed to update.
The Jones vector Es(t) constitutes a complete description of the optical signal, or more precisely of the signal's optical spectrum in the region of the local oscillator. This means that any parameter of the optical signal can be deduced from Es(t). Employing sampled coherent detection is more complex than direct detection, but has many benefits. Phase encoded modulation formats can be employed, such as BPSK and quadrature phase shift keying (QPSK), which offer better sensitivity than on-off modulation formats. Also polarization multiplexed formats can be employed, which offer twice the information capacity for a given bandwidth of electro-optic components and a given optical spectral bandwidth. The polarization demultiplexing operation is performed within the digital signal processor, so no additional optical components are needed for it. In a long fiber optic transmission system carrying high bit rate signals the optical fiber propagation effects, such as chromatic dispersion and polarization mode dispersion, distort the signals. With sampled coherent detection the propagation effects can be reversed within the DSP by applying an appropriate mathematical operation.
Finally, a key benefit of sampled coherent detection is that it is equivalent to passing the signal through a narrow optical filter centered on the local oscillator wavelength, so no narrow optical filter components are needed for WDM. The LO can be tuned in wavelength, which is equivalent to tuning the optical filter, and lower WDM channel spacings should be possible with sampled coherent detection than with any WDM implementation using passive optical filters. However, because sampled coherent detection is a means of observing a signal, the narrow channel spacing is not available in conjunction with a WDM channel add-drop. At the add-drop node the dropped channel must be extinguished before the add channel is inserted, and even if the narrowest available physical optical filter were used, it would work correctly only if the neighboring channels were spaced farther away than if the only constraint were the ability to detect with sampled coherent detection. Hence it is desirable to have a method of dropping and adding a WDM channel which allows the same low channel spacing as with detection only.
D. Digital Signal Processing
The present invention utilizes digital signal processing (DSP). DSP is described in “Understanding Digital Signal Processing” by R. G. Lyons (Prentice Hall, 1996), herein incorporated by reference. A signal processor is a unit which takes in a signal, typically a voltage vs. time, and performs a predictable transformation on it, which can be described by a mathematical function. FIG. 4a shows a generic analog signal processor (ASP). The box 402 transforms the input signal voltage 401 into the output signal voltage 403, and may contain a circuit of capacitors, resistors, inductors, transistors, etc. FIG. 4b illustrates a digital signal processor. First, the input signal 401 is digitized by the analog to digital (A/D) converter 404, that is converted into a sequence of numbers, each number representing a discrete time sample. The core processor 406 uses the input numerical values to compute the required output numerical values, according to a mathematical formula that produces the required signal processing behavior. The output values are then converted into a continuous voltage vs. time by the digital to analog (D/A) converter 408. The connections 405 and 407 between the A/D and D/A converters and the core processor are typically implemented as parallel data connections, which is why they are drawn as grey strips in FIG. 4b as well as in some of the other figures.
Digital signal processing can be a better solution than analog signal processing for a task because the signal processing operation can be varied under programmable control and because operations can be performed that would require too much complexity if done by ASP. The examples presented herein refer to a single digital signal processor, but in fact the DSP may be made up of several processors that communicate with one another, and they do not have to be co-located.
E. Secret Communications
Additionally, there are applications for secret communications, where the information being transmitted is not available to someone who has access to the transmission system at an intermediate location. Optical transmission is not inherently secure. An eavesdropper who taps off some of the optical signal power may observe the same signal as the intended recipient. If the signal is a digital signal the eavesdropper can reconstruct the same digital sequence as the intended recipient. Using a phase or polarization encoded format will stop the eavesdropper receiving the information if direct detection is utilized, but not if coherent detection also is used.
Many encryption methods are available that operate on the digital data, as discussed in “Applied cryptography” by B. Schneier (Wiley, 2nd ed., 1996). Clearly, one of these encryption methods could be used prior to optical transmission and the corresponding decryption method after detection, to make the optical transmission link secure. Most encryption methods employ a secret key known to the intended recipient, but not to an eavesdropper. The key is a piece of data typically shorter than the message it encodes. The security of the code is maintained even if the eavesdropper knows the design of the code, provided he does not know the key being used by that particular recipient. If the length of the key is m bits, then the eavesdropper can break the code by trying all 2m possible values of key. m is chosen as a large value such that an exhaustive key search would require an unreasonable amount of time and effort. However, for many codes whose design has been published a mode of decrypting the code has been found, often after years of research, which requires less effort than trying 2m key combinations. Thus it is desirable to find new encrypting methods which inherently require an unfeasibly long time and/or large resources to break.
Transmission in the optical domain offers some features which allow encryption with a higher level of security than using the data domain. For example, the optical domain has a much higher information carrying capacity than an electrical cable or link. Frequency hopping (FH) is a method that has been used for some time in secure radio transmission, and can be applied to the optical domain as described in “Secure optical communications utilizing PSK modulation, polarization multiplexing and coherent homodyne detection with wavelength and polarization agility,” by A. Salamon et al. (Military Communications Conference 2003 (MILCOM 2003), vol. 1, p. 274-282, 2003). The optical carrier frequency ωs is changed suddenly over time according to a frequency hop plan derived from the secret key. The intended recipient who knows the frequency hop plan can tune his receiver to the correct channel ωs(t) as it changes with time, and recover the signal that was sent. An eavesdropper must listen to all possible channels to be able to assemble the signal correctly. Security is improved by frequency hopping, but it can be defeated by an eavesdropper who has equipment to listen to all channels.
Thus, there is a need for an optical domain encryption method which can be implemented cost effectively for the intended information recipient but which is unfeasible to overcome by an eavesdropper.